This invention relates generally to Surface Acoustic Wave (SAW) filters and, more particularly, to the elimination of undesirable time spurious signals such as electromagnetic feedthrough and acoustic echoes in SAW filters.
Triple transit echoes and electromagnetic feedthrough associated with SAW filters represent well-known problems in the design of communications systems. Triple transit echoes are produced when the acoustic wave generated by an input transducer is first reflected and regenerated by the output transducer, and then is again reflected and regenerated by the input transducer. Actually, such regenerations and reflections of the SAW back and forth between the input and output transducers result in fifth, seventh and higher order transit echoes in addition to triple transit echoes. The resulting echoes degrade filter performance by creating both group delay and pass band ripples.
Many successful techniques have been developed to reduce the effects of triple transit echoes in SAW filters. Specifically, single phase, two-phase, three-phase and four-phase unidirectional transducers are often used to reduce the effects of triple transit echoes. Single-phase unidirectional transducers (SPUDT) are commonly used because such filters are easily fabricated and require only simple matching circuits at the input transducer and output transducer, respectively. Distributed Acoustic Reflection Transducer (DART) SPUDT, Electrode Width Controlled (EWC) SPUDT, Group SPUDT (GSPUDT) and Dithered SPUDT (DSPUDT) are examples of different types of SPUDTs.
SPUDT SAW filters are widely used in mobile phone and satellite communications systems because of their superior channel selectivity and relatively low insertion loss. However, SPUDT filter performance depends heavily on the stability of the matching circuits. Therefore, decreases in triple transit suppression (TTS), and increases in group delay and pass band ripples due to matching component (resistor, inductor and capacitor) value variations and temperature-induced mismatch in the matching circuits adversely affects the performance of such filters. It can also be labor intensive to fine-tune the matching circuits in order to meet the requirements for the high performance SAW filters used in satellite communications.
Two identical or similar cascaded filters are commonly used in the above-mentioned systems. In order to overcome the above limitations, these filters are generally overdesigned in order to meet the cascaded requirements because the cascaded group delay and pass band ripples associated with the triple transit echo in general will double, and the cascaded TTS will degrade by 6 dB with respect to individual filter responses. In general, TTS worsens as the insertion loss of a SAW filter is reduced. As a result, the insertion loss of the SPUDT filter is purposely designed to be higher than the filter""s capability in order to enhance TTS. However, this overdesign increases the overall cost of implementing the cascaded filters.
Electromagnetic (EM) feedthrough is the EM energy coupled directly from the input transducer to the output transducer and from the input matching circuit to the output matching circuit. Since the EM waves travel near the speed of light, the feedthrough usually shows up as a time spur near time=0 seconds. The feedthrough is an undesired signal that degrades filter performance by creating both group, delay and pass band ripples in the pass band, and that reduces the ultimate rejection outside the pass band regions in the frequency domain. The problem becomes very prominent for high frequency SAW filters since the input and output transducers and the input and output matching circuits are placed much closer to each other since the filter geometry is inversely proportional to the operating frequency.
There are many methods for reducing feedthrough. Some examples are 1) carefully designing the filter packages to electrically isolate the input transducer and input matching circuit from that of the output side; 2) using balanced transformers in input and output matching circuits; 3) strategically grounding the transducers to the package; and 4) Inserting a metallic ground bar between the input and output transducers. Methods 3) and 4) are simple to implement; however, the feedthrough cannot be fully suppressed, especially when the operating frequency is high. Methods 1) and 2) are more effective methods for reducing feedthrough; however, these methods increase the overall cost of the filter package and the complexity of the matching circuits. When two such filters are cascaded, the cascaded feedthrough level will be 6 dB worse than the feedthrough suppression of each individual filter. This degradation in feedthrough suppression becomes problematic in the above-discussed satellite and mobile phone applications in which feedthrough suppression is crucial.
In addition, reflections from the edges of the transducers in a SPUDT SAW filter will cause undesirable time spurs in the time domain response, especially for filters built on very strong coupling material like lithium niobate. When two such filters are cascaded, spur suppression will be 6 dB worse than the spur suppression of each individual filter. This degradation in spur suppression becomes problematic in the above-discussed satellite and mobile phone applications in which spur suppression is crucial.
Therefore, it is an object of the present invention to provide a surface acoustic wave filter system that eliminates group delay and pass band ripples associated with the time spur while at the same time is capable of maintaining time spur suppression without the need for costly system overdesign.
It is a further object of the present invention to provide a surface acoustic wave filter system in which the transducers of a first filter are offset from the transducers of a second filter by a predetermined amount with respect to either the time or frequency domain to eliminate group delay and pass band ripples.
It is another object of the present invention to provide a surface acoustic wave filter system in which a perturbation region located between the input and output transducers of a first filter is physically different from a perturbation region located between the input and output transducers of a second filter by a predetermined amount to eliminate group delay and pass band ripples.
It is a further object of the present invention to provide a surface acoustic wave filter system in which the transducers of a first filter are offset from the transducers of a second filter by a predetermined amount with respect to either the time or frequency domain, and in which a perturbation region located between the input and output transducers of a first filter is physically different from a perturbation region located between the input and output transducers of a second filter by a predetermined amount, to eliminate group delay and pass band ripples.
In addition, it is an object of the present invention to provide a cascaded surface acoustic wave filter that eliminates undesirable time spurs in the time domain response caused by reflections from transducer edges.
In view of the above, the present invention provides a cascaded SAW filter system in which two SAW filters are electrically cascaded in series to cancel time spurious signals. The first filter consists of one input transducer and one output transducer built on a piezoelectric substrate. A perturbation region may or may be not present between the two transducers of the first filter. The mth transit echo (where m is an odd number greater than 1) or feedthrough associated with the first filter is at a time TD (TD will be negative for the case of feedthrough) away from the main response in the time domain and has an associated frequency response with, a center frequency fo. The second filter consists of one input transducer and one output transducer built on a piezoelectric substrate that can be the same, or different, type of material as that of the first filter, and that can be fabricated either on the same substrate as that of the first filter or on a separate substrate. A perturbation region may or may be not present between the two transducers of the second filter. The mth transit echo or feedthrough associated with the second filter is at a time TDxe2x80x2 (TDxe2x80x2 will be negative for the case of feedthrough) away from the main response in the time domain, and has an associated frequency response with a center frequency foxe2x80x2 that is similar to, but typically slightly offset from, the center frequency fo of the first filter.
In the cascaded SAW filter system of the present invention, the group delay and pass band ripples associated with the time spur of the second filter cancel the group delay and pass band ripples associated with the similar time spur of the first filter because 1) the input transducer of the second filter is offset from that of the first filter; 2) the center frequency of the second filter is offset from that of the first filter; 3) the perturbation region of the first filter is different from the perturbation region of the second filter or 4) a combination of 1), 2) and 3) so that the group delay and pass band ripples between the two filters are 180xc2x0 out of phase and TD-TDxe2x80x2=(n+xc2xd)/fo, where n is an integer greater than or equal to zero. The associated time spur of the cascaded response will also be canceled. In particular, if the input transducer and the output transducer of the first filter are equal to those of the second filter, the cascaded mth transit echo or feedthrough can be cancelled by either offsetting the input and output transducers of the second filter by xcex(n+xc2xd)/(mxe2x88x921) where xcex=v/fo and v=the propagation velocity of the surface acoustic wave, m=2 for feedthrough and m is odd and greater than 1 for the mth transit echo, or by offsetting the center frequency foxe2x80x2 of the second filter by (n+xc2xd)/TD from the center frequency fo of the first filter.
A similar technique can be used to cancel the reflections at the ends of the transducers by adding extra dummy fingers at the ends of the transducers of the second filter so that the edges of the transducers of the second filter are approximately (n+xc2xd) xcex/2 from the respective edges of the transducers of the first filter.